1. Field of the Invention:
The present invention relates to a stepping motor for printers or the like and, more particularly, to a PM stepping motor.
2. Description of the Prior Art:
A stepping motor, as is generally known, is used as a driving source for moving the print head of a printer or as a driving source for feeding a printing sheet on a printer.
Referring to FIG. 5 showing a PM stepping motor for such purposes, there are shown a rotor shaft 2, a rotor 1 consisting of a collar 3 coaxially fixed to the rotor shaft 2, and a permanent magnet block la having magnetic poles arranged in divisions along the circumference thereof and fixedly fitted on the collar 3, inner yokes 5 and 6 and outer yokes 4 and 7. The inner periphery of each of the yokes 4, 5, 6 and 7 is bent inside to form n/4 pieces of pole pieces 8 (n is the total number of pole pieces of the yokes 4, 5, 6 and 7). The total number n of the pole pieces 8 is the number of steps for one turn of the permanent magnet block 1a. Further shown in FIG. 5 are a first driving coil 9, a second driving coil 10, a base plate 11 fixed to the outer yoke 4 by spot-welding or the like, a flange 12 fixed to the outer yoke 7 by spot-welding or the like, bushes 13 and 14 fitted in the respective central portions of the base plate 11 and the flange 12, respectivley, for rotatably supporting the rotor shaft 2 therein, and a shim 15 mounted on the rotor shaft 2 to take up the axial play of the rotor shaft 2.
As shown in FIG. 6, the permanent magnet block 1a has magnetic poles formed in the periphery thereof in circumferential divisions. As shown in FIG. 7, the inner yokes 5 and 6 are combined with positioning lugs (not shown) or positioning holes (not shown) formed in the respective inner circumference thereof coinciding with each other and are fixed together by spot-welding or the like so that the phase difference between the pole pieces 8 of the inner yoke 5 and the pole pieces 8 of the inner yoke 6 is an electrical angle of .pi./2 as shown in FIG. 8.
Recent progressive reductions in the cost, and size of printers and improvements in their reliability, require the compaction, reduction in thickness and improvement in performance of stepping motors to be incorporated into printers. The performance of a PM stepping motor can readily be improved by employing a rare earth magnet instead of the conventional ferrite magnet. However, the employment of a rare earth magnet increases the cost of a PM stepping motor. The performance of a PM motor can also be improved by driving the PM stepping motor by the bipolar driving system instead of the unipolar driving system. However, the employment of the bipolar driving system increases electrical parts, such as transistors, of the driving circuit resulting in an increase in the cost of the PM stepping motor.